Why did the United States enter World War I, and what effect did its entry have on the war?

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When World War I first broke out, the United States charted a path towards neutrality. Regardless, this did not prevent it from being pulled into the war.

The thing I would suggest to keep in mind is that, while the United States did not enter the war until 1917, the path was being laid well before. In 1915, a German submarine sank the Lusitania, spurring outrage and anti-German sentiment throughout the United States. Consider also the impact of the Zimmermann Telegram and the revelation that Germany was making overtures to draw Mexico into the war (along with its willingness to assist Mexico in acquiring territorial concessions from the United States in the event of victory) should the United States intervene. Meanwhile, be aware that Germany was resuming its use of unrestricted submarine warfare.

Thus, in April, 1917, the US entered the war. Keep in mind the sheer scale of destruction and devastation that World War I visited upon Europe. By this point, both sides were exhausted and near to the point of collapse. Having the United States intervene at such a late stage proved decisive. By the time the war's conclusion, roughly two million American troops had arrived foot in Europe. To this, one might add the role that the American navy played in supporting the British blockade of Germany. Also note the importance of United States economy and the impact of its production capabilities (agricultural, manufacturing, etc).

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The American people were overwhelmingly against joining World War I, but ultimately entered the conflict based on two events- the sinking of the Lusitania, and the interception of the Zimmerman Telegram. 

German U-boats attacked and sunk the British ocean liner Lusitania in 1915, resulting in the death of 120 American citizens.  While this event didn’t lead to a direct declaration of war, it did escalate tensions between the US and Germany and helped sway popular opinion against the Germans.  Later the British intercepted a telegram from the Germans (The Zimmerman Telegram) that would ultimately push the United States into a declaration of war.  The telegram from Germany urged Mexico and Japan to declare war on the United States to divert her attention from Europe.  Once Germany was victorious in Europe it would aide its two new allies in defeating the Americans.  Mexico was also promised that it would regain much of the territory it lost to the Americans in the Mexican-American war.

As a result of these events President Truman successfully petitioned Congress for a declaration of war, and officially entered the First World War on April 6, 1917.

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